First Observation of the DecayB0→D*+D*−

We have observed four fully reconstructed B{sup 0}{r_arrow}D{sup {asterisk}+}D{sup {asterisk}{minus}} candidates in 5.8{times}10{sup 6} {Upsilon}(4S){r_arrow}B{ovr B} decays recorded with the CLEO detector. The background is estimated to be 0.31{plus_minus}0.10 thinspthinspevents. The probability that the background could produce four or more signal candidates with the observed distribution among D{sup {asterisk}+} and D{sup {asterisk}{minus}} decay modes is 1.1{times}10{sup {minus}4} . The measured decay rate, B(B{sup 0}{r_arrow}D{sup {asterisk}+}D{sup {asterisk} {minus}})=[6.2{sup +4.0}{sub {minus}2.9}( stat){plus_minus}1.0(syst) ]{times}10{sup {minus}4} , is large enough for this decay mode to be of interest for the measurement of a time-dependent CP asymmetry. {copyright} {ital 1999} {ital The American Physical Society}